1. Field of the Invention
The present invention relates to a radiation image detecting device and a control method thereof.
2. Description Related to the Prior Art
A radiation imaging system, for example, an X-ray imaging system is constituted of an X-ray generation device and an X-ray imaging device. The X-ray generation device includes an X-ray source for applying X-rays to a patient's body, a source control unit for controlling the operation of the X-ray source, and an irradiation switch for inputting an X-ray irradiation start command. The X-ray imaging device includes an X-ray image detecting device for detecting an X-ray image in response to the X-rays transmitted through a patient's body part to be imaged, and an imaging control unit for controlling the operation of the X-ray image detecting device.
In the X-ray image detecting device, a flat panel detector (FPD) becomes widespread recently as an X-ray detector, instead of an X-ray film or an imaging plate (IP). The FPD has a matrix of pixels, and each pixel accumulates signal charge the amount of which depends on the amount of the X-rays incident thereon. The FPD detects an X-ray image, which represents image information of the patient's body part to be imaged, by accumulating the signal charge on a pixel-by-pixel basis, and outputs the X-ray image as digital image data.
There is practically used a portable X-ray image detecting device (hereinafter called electronic cassette) that has the FPD contained in a rectangular parallelepiped case. The electronic cassette is attachable to an imaging support designed for a film cassette or an IP cassette when used, besides being put on a bed or hand-held by a patient himself/herself. The electronic cassette is sometimes taken out from a hospital to the bedside of a home-care patient, an accident scene, or a natural disaster scene to perform radiography there without using the imaging support.
Conventionally, an operation signal generated from the irradiation switch is sent to both the source control unit of the X-ray generation device and the imaging control unit of the X-ray imaging device as a synchronization signal for indicating the start of X-ray irradiation. This allows the synchronization between the start of X-ray emission from the X-ray source and the start of signal charge accumulation in the X-ray image detecting device. To send the synchronization signal, the X-ray generation device and the X-ray imaging device have to be electrically connected to each other. If the X-ray generation device and the X-ray imaging device are manufactured by different makers and have incompatible connection interfaces (specifications of a cable or connector, format of the synchronization signal, or the like), it is necessary to newly prepare another interface compatible therebetween.
To solve this problem, there is proposed a technique in which the X-ray image detecting device detects the start of X-ray irradiation by itself for synchronization with the X-ray generation device without receiving the synchronization signal, in other words, without the electrical connection between the X-ray generation device and the X-ray imaging device (refer to U.S. Pat. No. 6,797,960 corresponding to Japanese Unexamined Patent Application Publication No. 2002-543684). According to this technique, bias current of the FPD, that is, an output value of a non-detection area of the FPD on which the X-rays not-transmitted through the patient's body part are incident is detected. Then, a differential value of the bias current is compared to a threshold value, to detect the start of X-ray irradiation.
Generally speaking, an output of an electrical component is susceptible to noise due to an internal factor of the electrical component itself or an external factor such as an ambient environment. Of course, the X-ray image detecting device equipped with a lot of electrical components is no exception. In the X-ray image detecting device, noise occurs by slight impact or vibration when the patient or a radiological technician unintentionally bumps thereon, for example. Such noise causes the X-ray image detecting device to malfunction. If a signal for detecting the start of X-ray irradiation has such noise, the X-ray image detecting device possibly makes a wrong detection of the start of X-ray irradiation, in spite of the fact that the X-ray image detecting device is not irradiated with the X-rays. The wrong detection makes the X-ray image detecting device execute unnecessary operation, and wastes power consumption. In addition, the radiological technician and patient need to wait for the termination of operation, and possibly miss a perfect shooting opportunity.
Worse yet, devices connected to the X-ray image detecting device, including the imaging control unit and a console used for a setup of imaging conditions, sometimes operate in response to the wrong detection, as if radiography has been actually carried out. In this case, burdensome operation e.g. reset of the imaging conditions is required, and affects a workflow of the radiological technician. Also, the risk of medical malpractice arises such that an inappropriate image obtained by the malfunction of the X-ray image detecting device may be transferred to a radiologist, or a setting error of the imaging conditions causes the patients to mix up.
A method described in the U.S. Pat. No. 6,797,960 is sensitive to the noise, because the bias current of the FPD is detected. When the bias current fluctuates by the noise, the X-ray image detecting device possibly makes the wrong detection of the start of X-ray irradiation. In spite of this fact, the U.S. Pat. No. 6,797,960 does not describe measures against the wrong detection due to the noise.